GB2222907A - "laser diode pumped solid state laser" - Google Patents

"laser diode pumped solid state laser" Download PDF

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Publication number
GB2222907A
GB2222907A GB8922797A GB8922797A GB2222907A GB 2222907 A GB2222907 A GB 2222907A GB 8922797 A GB8922797 A GB 8922797A GB 8922797 A GB8922797 A GB 8922797A GB 2222907 A GB2222907 A GB 2222907A
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laser
rod
cavity
output
frequency
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GB8922797D0 (en
GB2222907B (en
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Thomas Michael Baer
Mark Stanley Keirstead
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Newport Corp USA
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Spectra Physics Inc
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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/05—Construction or shape of optical resonators; Accommodation of active medium therein; Shape of active medium
    • H01S3/08—Construction or shape of optical resonators or components thereof
    • H01S3/08018—Mode suppression
    • H01S3/08022—Longitudinal modes
    • H01S3/08031—Single-mode emission
    • H01S3/08036—Single-mode emission using intracavity dispersive, polarising or birefringent elements
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/09—Processes or apparatus for excitation, e.g. pumping
    • H01S3/091—Processes or apparatus for excitation, e.g. pumping using optical pumping
    • H01S3/094—Processes or apparatus for excitation, e.g. pumping using optical pumping by coherent light
    • H01S3/0941—Processes or apparatus for excitation, e.g. pumping using optical pumping by coherent light of a laser diode
    • H01S3/09415—Processes or apparatus for excitation, e.g. pumping using optical pumping by coherent light of a laser diode the pumping beam being parallel to the lasing mode of the pumped medium, e.g. end-pumping
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/10—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating
    • H01S3/106—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating by controlling devices placed within the cavity
    • H01S3/108—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating by controlling devices placed within the cavity using non-linear optical devices, e.g. exhibiting Brillouin or Raman scattering
    • H01S3/109—Frequency multiplication, e.g. harmonic generation
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/02—Constructional details
    • H01S3/025—Constructional details of solid state lasers, e.g. housings or mountings
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/05—Construction or shape of optical resonators; Accommodation of active medium therein; Shape of active medium
    • H01S3/08—Construction or shape of optical resonators or components thereof
    • H01S3/086—One or more reflectors having variable properties or positions for initial adjustment of the resonator
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/10—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating
    • H01S3/10061—Polarization control
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/10—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating
    • H01S3/102—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating by controlling the active medium, e.g. by controlling the processes or apparatus for excitation
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/10—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating
    • H01S3/11—Mode locking; Q-switching; Other giant-pulse techniques, e.g. cavity dumping
    • H01S3/1123—Q-switching
    • H01S3/115—Q-switching using intracavity electro-optic devices
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/10—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating
    • H01S3/11—Mode locking; Q-switching; Other giant-pulse techniques, e.g. cavity dumping
    • H01S3/1123—Q-switching
    • H01S3/117—Q-switching using intracavity acousto-optic devices

Description

1 k PATENTS ACT 1977 Agents Ref: Q4303GB/ALM/mkf Description of Invention
"Laser diode pumped solid state laser" 2'1- 2 2 9 0 7 THIS INVENTION relates generally to lasers, and more particularly to solid state lasers such as Nd:YAG lasers.
A large number of diff-erent kinds of solid lasers have been discovered, distingui shed from one another by host material, by active lasing. ions with which the host IS doped, and by output characteristics. Of these, mainly ruby, Nd:YAG and 'Jd-doped glass laser systems are of major Importance in industrial and laboratory environmen-.s. They a 's processing re jart-cularl,., useful for materia.L applications which include drilling, welding, cut-.ng and 0 scrilbing.
A..;----e variety cf Nd:-_= lasers and industrial systems currentlv manufactured. Their usefulness ..ersnt- llty is due in part 'UG tIlle fact oerated in a number of difierent modes.
and _at the...
1 However, Nd-YAG lasers have proved to be relatively inefficient and have relatively short lifetimes due to Lations of their pumping sources which are typically arc 1 i m. i 1. or incandescent lamps, or light-emitting diodes.
Pumping by are or incandescent lamps is undesirable due to limited lifetimes. The lamps themselves have lifetimes of a few hundred hours and need periodic replacement. Moreover, they generate unnecessary and damaging ultraviolet radiation which tends to degrade the YAG material itself.
Pumping by light-emitting diodes is undesirable c CD because of limited power and focusabi','-- low ity and efficiency. The wavelength of the emitted light is very broad and does not match the Nd:YAG absorption line. Additionally, light-emitting diodes have a broad emilssion spectrum which provides inherent limitations when IL-Ihey are utilised as pumping sources for Nd:YAG lasers.
Exemplary Nd:"_fAG lasers pumped by those sources are disclosed by: F. W. OsICIermayer, jr., App-. ?Iiys.Lett., Iro-. 'lo. 3 (1971) p. 93; N.P. Barnes, J. Appl. Ph%rsics.
18, ', 7ol. 44, 'No. 1 ( 1973) D. 230: R. B. Chesier and D.A.
Draegert, Appi. Phys. Lett.. Vol. 2-':'), No. 0.
2 R.. B. Allen and E. j. Scalise, Aippl. Phys. Lett- "0.. No. 6 (1960) p. 18,15: and ".4. Culshaw, T. - -7 I J. Kanneland and,. E. Peterson, j. Quant. Elecz;_., Vol. QE-10, No. 2 (, 97L,) 21-73.
However. there ex-----t-= a need for a mere ef-J-4ent longer life Nd:YAG laser for low to high power applications.
need also exi-sts a frequency-doubled '1d:-,"-!'.- is efficient and suitable for which has a long 1. a:)i)l-icat.4ons in the visible light ranoe as well as other waveleneths. There is also a need for a laser witin low "urther exists for a laser with a amDlitude noise. A need 'L LI pulsed output. I'L-1 would also be desirable to produce a family of lasers using other neodymium-doped or other rare earth doped solid state materials in addition to Nd:UG (hereinafter referred generally to as RE:solid).
According to the present invention, there is provided a high efficiency, laser diode pumped solid state laser, comprising: a neodymium or other rare doped solid laser rod having front end and back end; a housing with means holding the laser rod in fixed position in the housing with its front end forward; a laser diode for pumping the laser rod, having an output frequency sufficiently matched to the rod to pump the rod, secured in the housing behind and in alignment with the rod; cavity means, including output coupler means, defining a laser cavity, mounted in the housing, with the laser rod within the cavity. 1 The invention also provides method for reducing or eliminating amplitude noise in the output beam of a laser diode array pumped neodymium doped solid laser, comprising positioning an amplitude noise suppression etalon in the 1 aser cavity.
A frequency doubler may be positioned within the Ilaser cavty to receive a suitably polarised output beam c.-^ the i laser rod to halve its wavelength and double itS ii. necessary polarization means are included in the cavL1..., 0 facilitate for polarizing the laser beam in order to L -i. U - eff-Lcient frequency doulbling.
the - addition to Nd:-fAG, other preferred materials for n r----; Include 'Id:YAP and Nd:YALC).
In preferred embodiments, particular features of the laser d-ode array pumped system of the invention are included for highly efficient and compact construction, as we-l as efficiency in laser pumping, frequenby doubling and polarization of the beam, suppression of amplitude noise, and pulsed output operation.
The present invention enables the provision of an intra-cavity frequency doubled RE:solid laser which allows efficient pumping by a high power laser diode array. The present invention also provides an expansion of the lasing volume to match the focused image of a laser diode array.
f Q 11 An intra-cavity waist is disleosed which provides effficient frequency doublin-. In a preferred f o 1 d e d c a v i t -v- L configuration, a pair of intra-cavity waists are provided.
Laser diode arrays provide a great deal of power despite the limited focusability of the output beam. With multi-strip arrays, e.g. having ten emitters in a row, each having an elliptical beam configuration, the compilation of the emitted beams adds uo to a rectano:ular geomet ---al beam which possesses too much spatial structure. Advantageously, embodiments of the present invention overcomes this,4isadvantage b-.1, orovidln.- a cavitv des, a to exzan-- the gned L' lasing volume to match the focused image of a lasser diode array. An intra-cavity waist is disclosed which provides efficient frequency doubl4na. in a referred 'Idecavii--,- CD p -, - fo- - - configuration. a pa4r of 4ntra-cavity waists are prov4ded.
0 1 __. - - U The invention is also advantageous in some applications Without frequency doubling, yielding an efficient near infra-red laser beam from low to high power.
In methods according to the invention a REE:solid laser rod 4.s pumped by a laser diode to produce m-n outp,.:-- -:n th=near infra-red range which may be doubled with intra-cavity frequency doubling to produce a visible beam.
of the beam is performed by the laser rod -itsel-' or else intra-cavity for efficient frequency doubling.
Amplitude noise is suppressed by means of an etalon placed in the cavity, or alternatively by a ring. cavity c - configuration, or a pair of quarter wave plates. Pulsed operation is obtained using a Q-switch.
In order that the invention may be readily understood. embodiments thereof will now be described, by way of example, with reference to the accompanying drawings, in f - 5 whi eh:
Figure 1 is a sectional longitudinal view of a laser diode Dumped solid state laser assembly embodying the invention including a laser rod, laser diode, laser cavity, housing, cooling device and other associated components, and showing (in dotted lines) the additional features of a frequency doubler, noise suppression etalon, and Q-switch; Fi.gures 2A, B, C are schematic sectional views of the system with various alternative means for polarizing the laser 1-,e-=,.; Figure 2D is a sche.niatic sectional view showing a system with etalon and Q-swit-ch; Eigure 3 is a graphic- representation of the laser beam - formed shape within the laser calvity, with a beam waist 'etween the laser rod and an output coupler at the fr- nt end the assembly, also sh--winc,:,, the position of the op -ao n:Dt to scale, "the laser system, indicating special surfaces of L khe rod 7Jj. g ' tudinal ure 5 is a schematic sectional view, in long- 4 orienzation, of a folded cavity configuration; and Figure 6 is a graDhic representation of the laser bearn shape within the folded cavity, with a pair of beam waists.
In the drawings, Figure 1 shows a neodymium-YAG laser assembly 10 in a longitudinal sectional view. The major components of the laser 10 are a neodymium-YAG laser rod 11 and a laser diode 12 toward the rear of the assembly. The assembly includes lenses 13 and 14 through which the laser 6 - diode beam passes en route to the laser rod 11, a L.-eauency doubler lo (dashed lines) at the output side of the laser rod, an outputcoupler 17 which comprises the front surface of a mirror) at the front end of the assembly, a heat sink 18 at the rear of the assembly, a Peltier cooler 19 between the diode 12 and the heat sink 18, and a housing 21, which may ccmprise front and rear housing components 22 and 2'-, to which all of these operating components are attached. Also included with the assembly are a temperature control 24 and a power supply 26.
-rical power to The power supply 26 delivers elect laser diode 12, causing it to emitt a laser diode beam 27 creating some waste heat which is removed by the ?elt cooller 10, and the heat sink 18. The temperature control 4s shown connected to the Peltier cooler 19 to regulate temDerature of the diode and to tune it by temperature the and ier to en the cor-rect-.,4ave- --th for pu-,,ip--ng of t,,e -AG laser rod 11. The laser diode array 12, which may be aGallium, aluminum arsenide (GaAl-lis, laser diode array, as identiLle as Model No. 2410 manufactured by Spectra Diode Labs of 3333 actured to Nort.h First Street, San Jose, California, is manur be close to the proper wavelength for excitation of the Md- Q YAS rod, but temperature control is requIred for preci-se ' the diode's output beam 27.
lltuningll ol In one preferred embodiment, the laser diode array 12 emits a beam substantially at.808 micron wavelength, the proper CJ l?' wavelength for pumping of the Nd-YAG rod 11. Such a laser "iciency of about 20%S.
diode has an eff As indicated somewhat schematically in the drawing, the diode 12 may be retained in the housing by a diode clamp 28.
A fixed lens mount 31 is secured in a portion of the housing, which may be a rearward end flange 32 of the 1 forward housing component 22, and retains the lens 13 in J U fixed position therein. The fixed lens 13 acts as a collimating lens to convert the diverging beam 27 from 1..Ielaser diode array 12 into substantially a parallel beam.
The collimated laser diode beam 27a then passes through the lens 14, which is a focusing lens, for focusing the beam into the back end of the Nd-YAG crystal 11. As indicated, the focusing lens 14 is adjustable, mounted on an adjustable lens spool 33 which is rotatable within a threaded bore as shown, to adjust the fore and.aft position of the lens 14. An opening 34 preferably is provided in the forward housing component 22 for access to the adjustable lens spool to rotate it via a series of holes 36 in the lens spooll.
Lhe focused, converging laser diode beam 27b enters the Md-YAG laser rod 11 and excites the neodymium atc=s in the rod to produce a laser beam in the near inflra-red ran.ce.
4. r p A laser cavit, for the Nd-YAG laser rod is defined between the output coupler 17, which comprises a partially:nirrc).-e,-i surface, and an opposing rear mirrcr ^ocated somewhere to the rear of the Nd-YAG rod 11. In one embo---,-rent o,-" the invention, the rear surface ---c, zf the --- J laser rod 11 itself is coated to be highly reflect.ve at ng as the rear mirror of the laser cavil--,,..
1.r)6 mic-ron, serv W A rod lhis is also indicated in Figure 4, showing the Nd-'_ U

Claims (3)

11 in enlarged view. It should be noted that the term '1=4-rrored" as used herein and in the appended Claims, includes partially mirrored. Forward of the Nd-YAG laser rod 11 is the intracavity frequency doubler 16 which preferably, but not necessarily, is included in the assembly 10. The emerging laser beam 41 from the Nd-YAG laser rod 11 passes through the frequency doubler 16 where its wavelength is halved, doubling its frequency. Preferably, the frequency doubler 16 is a 1 crystal which is a near-ideal frequency doubling element for this purpose, selected from a group including K17-11. LiNbO_ D crystal is a suitable and preferred and LiIO... A KT frequency , doubler, being an efficient doubling eleemenll in the wavelengths concerned with this invention. The power output of the KTP crystal frequency doubler increases approximately quadratically with increases in the 1.06 micron laser beam power, so that the efficiency of a system utilising this frequency doubler is much greater at high. powers than at low powers. The laser beam should be polarized within the laser cavity for maximising efficiency in frequency doubling. intra-cavity frequency doubler 16 only converts Incident - an axis polarized along a cert. light will pass through doubleer 16 along an or4A.".hocronal -x-s an-- C> W not be frequency converted. Therefore the incident laser "I" the -axis c--- beam should be polarized to coincide wif. doubler 16. This can 'Cle ac-- omDlLs'--ed in se-.reral ways. One preferred method embodying the present invention is to simply apply a transverse stress to the Nd:YAS rod!';! "ect of creating a beam polarizatic)n which. which has the efJL W the stress an,' is alon= the axis of the stress. The axis o. resulting beam polarization should be oriented relative to 0 the conversion axis of doubler 16 to maximize conversion. The transverse stressing of the laser rod 11 may be accomplished by a simple set screw or stressing screw 42 threaded into the housing component 22 as shown. Since it is important that the transverse stress on the laser rod be substantially constant, it may be beneficial to add a strong compression spring to the assembly including the set screw 42, for example a Belleville washer, between the set screw and the laser rod 11. Although this is not shown in Figure i - 9 1, a schematic indication of a Belleville washer 43 contacted by the set screw 42 is included in Figure 2A, with the force of the Belleville washer 43 applied to the side of the Nd-YAG rod 11 by a spacer member 44. Figures 2A, 2B and 2C show schematically the major components of the laser diode and Nd-YAG laser assembly, and erent systems for establish indicate three difl ing a In Figure 2A, as polarizat.on in the laser beam 41. discussed above, the transverse stressing of the Nd-'"I"! rod itself is illustrated. Figure 2B shows an alternative method wherein a quarter waveplate 46 is used, between the ront end mirrored sur_-ace 17. frequency doubler lb and the Figure 2C shows the use of a Brewster plate 47, i.e. a piece of alass oriented at Bewster's anale. It is imDo--,ant to the polarization within the laser cavity. control t A -her important feature of the invention re_'-=tes 1- no. beam shaping in the laser cavity as indicated in F-',=u,-es I and 22., ', th,rough 2C, the partially mirrored surface 17 at the output coupler prefferably is concave. It is also indicated in these f.gures and in Figure 4 that the front end surface -,:f the 'Id-YAO, laser rod 11 may be convexly curved. 7' h e curvature of the -front of the Nd-YAG rod, which nay be a si)her4--,a-- curvature of about 15 millimeters radf-us, in effect puts a lens in the laser cavity which tends to focus the rad-ation. Coooeratina with this lens in the shaping of the beam within the cavity is the output coupler mirror 17. 1he craph of Figure 3 shows generally the laser beam 41 in prof ile within the laser cavity. It illustrates beam shaping to form a beam waist 50, that is, a narrowed portion of the laser beam as it resonates within the laser cavity between the two mirrored surfaces. In the representation of Figure., the rear mirrored surface is assumed to be the "'at back surface 39 of the Nd-YAG laser rod. Varying of the radius of curvature of the lens surface ^ect the 48 at the front of the laser rod has been found to aft size of the beam waist 150. A tighter radius of curvature will produce a smaller waist which enhances the frequency doubling process. It has been found advantageous for efficiency of a laser embodying the present invention to reduce the beam waist 50 to the minimum diameter practicable relative to other design considerations, includin= permissible ranges of radius at the front end 48 of the laser rod. and to place the KTP frequency do,-,bl-i.-.,c crysta at the beam waist. A minimum practicable waist diameter ma- or the embodiment shown. be about UO microns L Another aspect of beam shaping in a laser embodying the - - U_ present invention relates to matching of beam volume of the resonating beam inside the YAw rod to the size of the laser 11 crys-na-.-cn of the diode beam exC-'t4Ln,z he 1.iU %,a!. Tihe com'- concave outDut cou---'er mirror 17 and the lens-sha:,,e--, end he ront of t..e with the back 3al of the r.----4 at the L YAG rod, mirrored, enables the beam size at a loca'L.4.on on tIne "Jgure 3, j4..e. within the YAG rod, to be adjus-.e,-' graph of F. to the appropriate volume. The beam focused from the laser diode inzo the _= crystal must overlap the bear. -,,o:,-u:-,e inside the laser rod, for excitation of the neod,;m4;.um atoms within the rod. The pumping volume must be generally th,e same as the lasing volume. If the laser beam volume within the YAG crystal is too small, the pump volume - from the laser diode beam does not match it well and this results in a reduction in the laser's efficiency. The combination of the lens-shaped end 48 on the laser rod, the output coupler mirror 17 and its radius of curvature, the distance back from the lens 48 to the rear cavity mirror 3.0 (preferably on the flat back end surface o. the YAG rod), which preferably is about 5 millimeters, and the placing, of the KjCP doubling crystal. at the beam waist _Z I- 1--0, which is of minimum practicable size, resulks in a highly efficient frequency doubled laser output. The radius of curvature of the concave mirror 17 at the outout coupler, in one preferred embodiment of the invention preferably is about 37 millimeters. The distance between this concave mirror and the forward end of the KTP crystal may be about 'l Mill_=eters. may be used. A K17P crystal of about 5 millimeters length From the rear of the KTP crystal back to the lens-shaoed front of the YAG rod may be about millimeters. As stated above, the YAG rod itself mal, be of about', I--millimeters length, with a 15 millimeter ra----us Cf curvature of the front-end lens 48. 22 !I. should be understood that the mirror locations shown and described herein are preferred, but may vary. For example, the rear mirror surface of the laser ca,.r-t-,, ma,: comprise a mirror placed somewhere behind the back sur-face 3.9 of t.-.e laser roj. .1 kl- laser d11 llith 'i,- ode array ou,.,.lped Nd-Y'"11G laser of 1t---he invention it has been found that for visible low power laser beam output. efficiencies of about may be achieved. For example, with about one watt power suppiled to the laser diode, which has an efficiency of about 20%, the laser diode output beam will! have a power of about 200 millwatts. In general at these pur.p levels the 1.06 micron output is approximate-ly --,OOP' of the diode laser output, so that the 1.06 micron output beam has a power of about 60 milliwatts. Thus an efficiency of approximately 5% is achieved for output at 1.06 micron. For efficiency frequency doubling the output coupler is coated to be highly reflective at 1.06 micron and highly transmissive at.532 micron. At 200 milliwatts pump levels the intra-cavity 1.06 micron intensity is approximately 10 watts. At this power level the doubling efficiency of the K-LP is sufficient to give approximately 10 minutes output at f I-%- 532 micron. -ts of At substantially higher power, for example 10 wat input to the laser diode, a 2-watt output diode excites the YAG rod to emit a laser beam of about 600 milliwatts. A't this higher power, the frequency doubling crystal is more efficient, and an output in the visible range of about 100 'ic4ency in milliwatts can be achieved. Thus, one percent efL a mediumpower visible laser is achieved. At high-power output, the Nd-YAG laser of the invention is considerably more efficient. For example, if 40 watts f 2 are input to the laser -'-ode, a laser beam ol. abou-k -'n e K watts is frequency doubled, and at this power 11, P frequency doubler converts nearly 100 percent of the 1. OCS put beam micron output light to visible. Thus, an out of over two wa'l'.s in the vissible range can be achieve,-, at uD to 51110 to 05, efficiency. T he system of the invention is also advantageous for -he near-infra-red range. In th's f producing a laser in 1. 1- I'orm of the invention the frequency doubler 16 (in dashed lines 4L s e 11 r.n. i n a t ed Thus, the ef f i ci ency of- L' he n gure ', 1 ff ciency system is limited only by the approximately 20% ef. of the laser diode, and by the approximately 30'J'9' e..l.-'ici4encv P of the Nd-YAG laser rod itself, for an overall efficiency oú nearly 6'115 regardless of power level. 0 In one form of such an infra-red laser, the ends of the Nd-YAG laser rod may form the two mirrors of the laser cavity. Thus, each end is partially mirrored, defining a cavity within the rod itself. An extremely efficient nearinfra-red laser thereby results, e.ven more compact than the system shown in Figure 1, since the output coupler is integrated with the laser rod.
1. A method for reducing or eliminating amplitude noise in the output beam of a laser diode pumped neodymium doped solid laser comprising positioning an amplitude noise suppression etalon in the laser cavity. 1
2. A method for reducing or eliminating amplitude noise in the output beam of a 'Laser diode pumped neodymium doped solid laser substantially as hereinbefore described with reference to the accompanying drawings.
CD
1.
13 - A further problem that occurs in a miniaturized, laser diode pumped, intra-cavity frequency doubled Nd:YAG laser as previously described is the generation of amplitude noise, including large amplitude spikes, which present or limit use in applications requiring a highly stable or constant output. Although the short laser cavity results in longitudinal modes which are relatively widely spaced., the gain curve is generally sufficiently broad to pernit multiple longitudinal modes to oscillate in the laser cavity. The combination of these multiple modes produces ampl,.'.ude noise. In order to reduce or eliminate amplitude noise, an amplitude noise suppression etalon 52 is Pl-nced in the cavity normal to the beam, as shown in Figures 1, 2D,
3.
Alternatively, it may be possible to mode lock the 'Laser to reduce noise. Using etalon 52 causes the laser 'Vc oDerate in s_ingle mode which is quiet. An example of an et-nion 52 which can be used is an optical flat of about ^J.5 mm thickness. Since the beam waist 50 is not only the narrDwest. portion of the beam but the portion of '.-e bea where all the rays are parallel, it is preferred to place etalon at the beam waist 50 in order to reduce c.2tical losses. Since it is also preferred to place doubler 16 at :-C, etalon 52 can be placed adjacent doubler 16 as in Figure 3.
_o avoid the difficulty of placing two elements, doubler 16 and etalon 52, at beam waist 50, an alternate confi,c,.,,u,ration, folded cavity- 54, shown in Figure 5, iss preferred. Folded cavity 54 includes a concave folding mirror 56 which forms with rear mirror surface 39 of laser rod 11 the first arm of the laser cavity, and concave end mirror 58 which forms with folding mirror 56 the second arm of the laser cavity. Folding mirror 56 is a dichroic mirror which is highly reflective at the undoubled frequency and highly transmissive at the doubled frequency, and is used as the output coupling means for the visible light. Mirror 1.58 k 14 - is highly reflective at both frequencies. Frequency doubler 16 is placed in the second arm between mirrors 5b and 58 so that the laser beam produced by rod 11, with the proper polarization, is reflected by mirror 56 and passes through doubler 16. The frequency doubled beam is reflected back by mirror 58 to mirror 56 through which the beam is output. The frequency doubled radiation thus does not pass' back through the first arm to laser rod 11. An amplitude noise suppression etalon 52 can be placed in the fir-st arm between mirror 56 and laser rod 11. An illustrative beam profile within the folded cavity 54 is shown graphically in Figure M Cirst arm and a 6. A first beam waist 60 is produced in the L ser-ond beam waist 62 is formed in the second arm with the profile extending between mirror surface 39 and mirror 55 with an intermediate point 64 at mirror 5o. As previously described the beam width at laser rod 1 1 is matched to the laser diode pumping volu,-,ie. Doubler 16 is placed at wa- s-, 62 wh4le etalon 52 is placed at waist 60. -ypic-n-' dimensions of the folded cavity are a total length of about 0 100-130 mm; the radius of curvature of mirrors 58 is typically 37 mm; beam waist 60, 62 are typically less than 100 microns.
The primary cause of multi.-longitudinal mode operation W - in a Nd:YAG laser is spatial hole burning in the active medium. Several techniques exist for eliminating spa"t,.ia-, hole burning, includng utilising a ring laser cavitr geometry or placing the active medium between quarter wave plates, which are shown in W. Koechner, Solid State Laser.Engineering. (Springer-Verlag, New York, 1976), p. 226. Either of these techniques can be applied to the intracavity doubled laser system described herein, instead of using an etalon, and form additional aspects of the invention. By eliminating spatial hole burning the laser will lase with a single longitudinal mode and thus not suffer the mode instability and amplitude fluctuations R - described above. Utilising a ring laser cavity geomerty or a pair of quarter wave plates has the advantage that litt-ler power is lost when these elements are inserted in the cavity whereas us-ng etalons to force single mode operatio,r often results in a factor of two loss in power.
As previously described, in order to utilise the intracavity frequency doubler to generate a frequency doubled laser output, the output of the laser rod must be polarized to coincide with the proper axis of the doubler crystal-. When a non-birefringent material such as YAG (yttrium aluminum garnet Y 3 A1 5 0 12) used for the laser rod, a Do"ariza-.-on means within the cavity is required, as previousy shown. However, it is also possible to ut--!-ise a birefringent material for the laser rod; the output:f the birefringent laser rod will then be polarized, withcu'-1 the ^or Dolarization means, and the laser rod and doubler need L er.,szal can be properly aligned for effective frequenc... conversion. One suilable birefringent matrial for the laser A d is (yttrium lithium fluoride YLiF4 accor,--,ngly, --s also a preferred material for the invent--on, -n ad-it-on to 'Md:YAG. Other non-birefringent materials such as (yttrium aluminum phosphate) and birefringent materialls. such as Nd:YAL0 can also be utilised. Additional "c need doped or other rare earth doped solicstat materials may also be utilised as long as the lasing ion has an absorption range which matches the laser diode output.
-he present invention encompasses the use of these ;ve materials, both non-birefringen'. and alternat birefringent, in a manner similar to that described with reference to Nd; YAG, without the polarization means for birefringent materials, to produce a family of miniaturized, laser diode pumped, intra-cavity frequency doubled and nonfrequency doubled solid state lasers.
In some applications, pulsed laser outputs are desired.
The lasers as previously described generally operate in continuous (Cw) mode. Although it may be possible to produce a pulsed laser output by pulsing the laser diodes which pump the laser rod, the preferred method of producing pulsed output is by Q-switching. As shown in Figures 1 and 2D, a Q-switch 66, typically an acoustic-optic or electrooptic device, is positioned in the laser cavity. A Q-,9witch driver 68 is operatlvely connected to the Q-switch 66. IL n to allow a operation, the Q-switch turns the laser off population inversion to build up as the laser rod is pumped by the laser diode. The Q-switch is then turned off, pr-_ducing a hIgh energy pulse as all t.he stored energy in 0J. 0 the laser cavity is released in a short time. The pulse width is determined by the Q-switching f.requency. For pulsed operation Y1F may be the preferred material since it stores more enerl--,jr flabut double) than v-AG. AMpl--!,.U--e noise is no'. a problem _for pulsed operation. Both frequencv. doubled and frequen-c., u,-.,.-'oubled lasers can be pulsed As an examplee, a Ilaser producing 80-100 mw!R can be frequenc- W doubled and Q-switched to produce 50 mw average power at 1OU0 kHz creen oulses.
0 CLAIMS:
GB8922797A 1985-12-19 1989-10-10 Method of generating a laser beam having reduced or eliminated amplitude noise Expired - Lifetime GB2222907B (en)

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FR2592238A1 (en) 1987-06-26
US4656635A (en) 1987-04-07
GB2184596A (en) 1987-06-24
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JP2614440B2 (en) 1997-05-28
GB8922797D0 (en) 1989-11-22
GB2222907B (en) 1990-08-22
FR2592238B1 (en) 1995-03-31
GB2184596B (en) 1990-07-11
JPS62189783A (en) 1987-08-19
DE3643648A1 (en) 1987-07-16

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